Lecture 344/24/06

Entropy (Measurement of disorder) Related to number of microstates ∆S universe = ∆S system + ∆S surroundings 2 nd Law of Thermodynamics Entropy of the universe increases with spontaneous reactions Reversible reactions vs. Irreversible reaction

3 rd Law of thermodynamics S = O at O K S° - entropy gained by converting it from a perfect crystal at 0 K to standard state conditions

Degrees of freedom translational motion molecules in gas > liquid > solid vibrational motion movement of a atom inside a molecule rotational motion rotation of a molecule

Entropy trends Entropy increases: with more complex molecules with dissolution of pure gases/liquids/solids with increasing temperature with increasing volume with increasing # moles of gases

Which has higher entropy? dry ice orCO 2 liquid water at 25°Corliquid water at 50°C pure Al 2 O 3 (s)orAl 2 O 3 with some Al 2+ replaced with Cr 3+ 1 mole of N 2 at 1 atmor1 mol of N 2 at 10 atm CH 3 CH 2 CH 2 CH 3 (g)or CH 3 CH 3 (g)

Is the reaction spontaneous?

Gibbs Free Energy ( ∆G) ∆G° = ∆H° - T∆S° ∆G = ∆H - T∆S ∆G° = Σn∆G f ° (products) - Σn∆G f ° (reactants)

Gibbs Free Energy ∆G = ∆H - T∆S ∆H∆S-T∆S∆G spontaneous? example -+ 2O 3 (g)  3O 2 (g) +- 3O 2 (g)  2O 3 (g) -- H 2 O (l)  H 2 O (s) ++ H 2 O (s)  H 2 O (l)

Gibbs Free Energy (∆G) and equilibrium R = J/mol-K

Example A hot plate is used to heat two 50-mL beakers at the same constant rate. One beaker contains 20.0 grams of graphite (C=0.79 J/g-K) and one contains 10 grams of ethanol (2.46 J/g-K). Which has a higher temperature after 3 minutes of heating?